The invention concerns an apparatus for producing seamless plastic tubes.
Apparatuses of that kind having an injection head with a distribution device with distribution passages extending perpendicularly to the axis of the mould cavity have the advantage that the injection head on the one hand can be designed so as to be of a short structural length and on the other hand can also be designed to be of small diameter, so that the distribution device which is preferably in the form of a plate can be arranged within the cross-section of the mould cavity. By virtue of the short structural length of the injection head, the feed passage for the molten material can be of relatively large diameter over a large part of its length, so that it is possible to operate with comparatively low pressures, but nonetheless sufficient molten material is fed to the annular gap or the annular gaps which are arranged in succession in the production direction.
An injection head of that kind is known from DE 198 35 189 A1. The distribution passages of that so-called plate injection head are in the form of passages which are branched in a tree-like configuration. To provide those passages, suitable grooves are formed in a distribution plate, the grooves being covered by way of a cover plate. A disadvantage here by virtue of the distribution passages which are branched tree-like is that the variability of the number of distribution passages or the variability of the degree of branching of the distribution passages is restricted to stages which form powers of 2, and thus a stepless variation is not possible. A further fundamental structural limitation arises out of the fact that the distribution passages, in accordance with the concept of DE 198 35 189, must each of exactly the same length in order to ensure identical flow paths, in order finally to guarantee a homogeneous composition for the plastic extrusion issuing at the annular gap.
In addition DE 27 52 932 discloses an apparatus with a plate injection head. In that apparatus the plate injection head has a distribution plate with radial grooves which are arranged substantially in a star shape and which, covered by a cover plate, form distribution passages which are radial in a star shape. The molten plastic material is fed to the distribution plate by way of an annular gap which is concentric with respect to the central axis of the mould cavity. The molten material is fed by way of the star-shaped distribution passages to the annular gap arranged at the outer periphery of the distribution plate, the annular gap having a turning distributor integrated therein. In order to achieve an improved distribution effect, the embodiment illustrated in DE 27 52 932 provides that the star-shaped distribution passages which extend from the inner annular gap open into an annular distribution passage, from which further radial distribution passages of an increased number issue in a radiating configuration. That annular distribution passage does not guarantee homogeneous, trouble-free distribution. In addition, disadvantages arise out of the fact that space for feeding the molten material and for possible supply conduits must be present exclusively on the central axis within the annular feed gap, in a concentric arrangement. A further disadvantage is that, in order to ensure flow paths which are each of equal length, the entire arrangement must be of a strictly symmetrical configuration, with respective passage portions of identical length, around the central axis.
U.S. Pat. No. 3,809,515 discloses an injection head which is admittedly not in the form of a plate injection head but which already has distribution passages extending perpendicularly to the central axis. The distribution passages are directed radially with arcuate end portions which open in the manner of a turning distributor into the outer annular gap. The variability in the degree of distribution is limited on the one hand by the radial orientation of the passages and on the other hand by the configuration of the turning distributor adjoining same. A further substantial limitation is that, in accordance with the concept disclosed in U.S. Pat. No. 3,809,515, the flow paths in the distribution passages must be of exactly the same length in order to ensure a homogeneous volume flow.
An injection head which is known from U.S. Pat. No. 3,743,456 and which is also not in the form of a plate injection head also already has distribution passages extending perpendicularly to the central axis. They are arranged in the manner of a cross and open directly into the outer annular gap. In this case also, the degree of distribution is limited as only a limited number of exclusively radial passages are structurally possible, in the cross-section involved.
The object of the present invention is to develop an apparatus of the kind set forth in the opening part of this specification, in such a way that a composition of maximum homogeneity of the jet of molten plastic material issuing at the annular gap of the injection head is achieved.